1. Field of the Invention
The invention relates to a method and a device for improved determination of the position of a user device in a mobile-radio network.
2. Discussion of the Background
The possibility of determining the position of user devices is an important property of modern mobile-radio networks. In this context, user devices can be mobile telephones, but especially also any computers, which are fitted with equipment for transmitting and receiving within a mobile-radio network. Various position-determining methods are available:
In the case of simple cell positioning, the position of the radio cell, onto which a user device is logged, is determined. The accuracy depends upon the size and shape of the radio cells. The smaller the radio cells, the more accurately the actual and the determined position will agree.
With the so-called TDOA method (TDOA=Time Difference of Arrival), the position of a user device in a mobile-radio network is determined on the basis of the difference between the arrival times of a signal transmitted from the user device at a plurality of different receivers. With two receivers in known positions, the possible transmitter positions are disposed on a hyperboloid. If a third receiver is present, the possible positions are disposed on a curve, which is formed from the intersection of two hyperboloids. An accurate position determination requires at least four receivers.
However, one disadvantage of the TDOA method is an inadequate accuracy of positioning, the possible causes of which may be found in the geometry of the receivers and/or the transmitter, in the time measurement of the receiver system, the synchronisation of the receiver systems or in the bandwidth of the transmitted impulses.
The so-called E-OTD method uses the already-existing GSM infrastructure, in order to determine the position of a user device. The position of a user device is determined by triangulation across three base stations, wherein the evaluation is based on the time difference upon the arrival of radio signals at the user device (for example, mobile telephone). These time differences are either transmitted from the user device to the network for position determination, or the user device receives additional data from the network and, from the latter, calculates its position itself. The method also functions inside buildings, but, with deviations up to 100 metres from the actual position, does not achieve the accuracy of GPS. Furthermore, user devices must be specially adapted for E-OTD.
One method with relatively high accuracy is the so-called AGPS method (Assisted GPS). This is a method for the transfer of provisional data via a mobile-radio network for a faster GPS positioning. With reference to the radio cell, which the mobile telephone uses, the approximate location of the mobile telephone is already known. This position can be determined more accurately through measurements of the signal delay times from the other mobile-radio masts disposed in the vicinity. Reception by at least three base stations is required in order to calculate the location accurately. If only two base stations are received, the location can be limited only to two positions.
This information can be used to narrow the search range for the satellite signals (identity of the visible satellites, approximate delay time, Doppler shift) and accordingly to accelerate the measurement.
With conventional GPS, the receiver has two tasks. It measures the arrival time of the signals and reads the data transmitted from the satellites, which contain, inter alia, path parameters and error corrections. In the case of A-GPS, the satellite data are read by reference receivers, which are set up in a stationary manner with good visibility with regard to the sky. Their exact position data have already been measured. Accordingly, the mobile receiver need only measure the arrival times of the satellite signals, which requires a level up to 30 dB lower.
Dependent upon the location, at which the actual calculation of the position is implemented, a distinction is made between network-based and terminal-based mode. In the case of the network-based A-GPS, a user device transmits the measured signal delay times of the respective satellite signals, and a server in the mobile-radio network calculates from these the exact position, which is finally transmitted back to the user device. In this case, the provisional data consist exclusively of the search-field parameters. In the case of the terminal-based mode, the user device receives only the satellite data and auxiliary data and calculates its position itself after the measurement.
One disadvantage of the A-GPS method is that it requires a receiver for the signal of the GPS satellites in the mobile user device, which generates additional costs. A further disadvantage is that, inside buildings, the GPS signals can be received only very poorly or not at all.